When a motor or a transformer is manufactured, first, a hoop-shaped electrical steel sheet is punched into a predetermined shape and hoop-shaped electrical steel sheets are laminated and fixed to thereby manufacture an iron core. Then, a copper wire is wound around teeth or/and the like to then be immersed in a varnish and has powdered paint sprayed thereon. Thereafter, the iron core has terminals for connecting the copper wire, flanges, bearings, and so on attached thereto and is fixed to a case.
In such a manufacturing process of the iron core, a facility restriction of a press or the like is large, and thus there is often a case that a lot of time is taken for the process in which the electrical steel sheet is punched into a predetermined shape.
Thus, for efficiently performing the punching process, the hoop-shaped electrical steel sheet is well prepared beforehand and the process of winding the copper wire after the punching is performed collectively, resulting in that an increase in efficiency is achieved.
In the case when the hoop-shaped electrical steel sheets are stored, it is general to use a depository for preventing rusting, but there is often a case that the hoop-shaped electrical steel sheets are left outside the depository for efficiently performing the punching process, and in this case, it is necessary to pay attention to rusting particularly.
Further, in recent years, a process base of the iron core is shifted to China and Southeast Asia, and thereby a cost reduction is generally performed. In such countries, there is often a case that factories are established under a wet environment rather than in Japan, and corrosion resistance under a wet environment is needed rather than in Japan.
Normally, on the surface of the electrical steel sheet used for the iron core of an electric apparatus, an insulating coating film for reducing eddy current loss is applied, and the insulating coating film is required to have coating film properties such as corrosion resistance, adhesiveness, a punching property, and heat resistance in addition to insulation performance.
In this insulating coating film, a mixture having an inorganic acid salt such as chromate or phosphate and an organic resin as its main component is generally contained. In recent years, due to the environmental concern, an insulating coating film containing no chromium is required.
With the progress of an increase in efficiency of the manufacturing process of the iron core of an electric apparatus, the insulating coating film of the electrical steel sheet is required to have more corrosion resistance than ever, and the improvement of the corrosion resistance under a wet environment in particular is required.
That is, it is sufficient for an insulating coating film for a conventional electrical steel sheet to have corrosion resistance enough to be able to suppress rusting in a depository before the punching process, but in recent years, corrosion resistance enough to prevent rusting even under the wet environment is required.
Further, it is possible to improve the corrosion resistance by applying the insulating coating film thickly, but there are problems that a space factor decreases and adhesiveness decreases.
Further, an electrical steel sheet having a coating material having a fluorocarbon resin as its main component applied on a surface thereof as the insulating coating film has problems that cost is increased and a varnish after the punching does not adhere to the electrical steel sheet.